Abstract: In 1997, NRC performed an evaluation of emergency planning for the evolutionary and advanced light water reactors, and indicated that no changes to emergency planning (EP) requirements were warranted in the existing technical framework. However, NRC also recognized if the lower probability of severe accidents or the longer time period between accident initiation and release of radioactive material was considered, the changes to EP requirements for advanced light water reactors might be warranted and the emergency planning zone (EPZ) sizing might be reduced. This means that if the accidents which probabilities are lower than a cut-off probability are not considered, there may be a significant impact on the EPZ sizing. AP1000 was selected as an example in this study and the probable impacts of different cut-off probabilities were studied by using MACCS code in accordance with the NUREG-0396 method. The results show that, only if the cut-off probability is higher than the occurrence probability of the accident which has relatively serious consequence and high probability, there will be a significant impact on the EPZ sizing for the advanced light water reactor.